There have been known light-emitting devices such as light-emitting diodes (LEDs) that use sapphire (α-alumina single crystal) as a monocrystalline substrate, with various types of gallium nitride (GaN) layers formed thereon. For example, light-emitting devices have been mass-produced having a structure in which an n-type GaN layer, a multiple quantum well (MQW) layer with an InGaN quantum well layer and a GaN barrier layer laminated alternately therein, and a p-type GaN layer are formed in a laminated manner in this order on a sapphire substrate. There have also been proposed multilayer substrates suitable for such an application. Patent Document 1, for example, proposes a gallium nitride crystal multilayer substrate including a sapphire base substrate and a gallium nitride crystal layer formed through crystal growth on the substrate.
It will be appreciated that when a GaN layer is formed on a sapphire substrate, dislocation is likely to occur because the GaN layer has a lattice constant and a thermal expansion rate different from those of the sapphire substrate, which is heterogeneous with respect to the GaN layer. Further, since sapphire is an insulating material, on the surface of which no electrode can be formed, it is impossible to form a vertically-structured light-emitting device including electrodes on the front and rear surfaces thereof. LEDs are hence receiving attention in which various types of GaN layers are formed on a gallium nitride (GaN) single crystal. Such a GaN monocrystalline substrate, which is of a homogeneous material with the GaN layers, allows for easy matching in the lattice constant and the thermal expansion rate and is expected to have an increased performance compared to using a sapphire substrate. Patent Document 2, for example, discloses a free-standing n-type gallium nitride monocrystalline substrate with a thickness of 200 μm or more.
However, monocrystalline substrates are generally expensive, though having only a small area. In particular, while cost reduction in manufacturing LEDs using a large-area substrate has been demanded, it is not easy to mass-produce large-area monocrystalline substrates, and the manufacturing cost may contrariwise further increase. Hence, an inexpensive material has been required that can be substituted for gallium nitride or the like of these monocrystalline substrates. There have been proposed polycrystalline gallium nitride free-standing substrates that meet such a requirement. Patent Document 4, for example, discloses a polycrystalline gallium nitride free-standing substrate composed of multiple gallium nitride-based monocrystalline particles that have a particular crystal orientation in approximately the normal direction. Patent Document 3 also describes a polycrystalline gallium nitride free-standing substrate composed of multiple gallium nitride-based monocrystalline particles that have a particular crystal orientation in approximately the normal direction, in which the crystal orientations of the gallium nitride-based monocrystalline particles measured through inverse pole figure mapping of the electron back scatter diffraction (EBSD) on the substrate surface are distributed in a manner inclined at various angles with respect to the particular crystal orientation, with an average inclination angle of 1 to 10 degrees.
Patent Document 3 provides an oriented GaN free-standing substrate in which polycrystalline particles of which the substrate is composed are controlled to have an inclination angle (tilt angle) of 1 to 10 degrees and a light-emitting device. This invention provides a polycrystalline gallium nitride free-standing substrate in which the surface can have a reduced defect density and a light-emitting device using the polycrystalline gallium nitride free-standing substrate to have a high luminous efficiency.
Patent Document 5 provides a high-resistance and low-defect Zn-doped GaN crystal and a method for manufacturing the crystal. The Na-flux method is used as a crystal growth method, in which Zn is added into a flux to cause the growth of a GaN single crystal.
Patent Document 6 provides a method for manufacturing a gallium nitride single crystal. In a mixed flux of Na and an alkali or alkali-earth metal, gallium and nitrogen are reacted with each other to manufacture a gallium nitride single crystal. However, Patent Documents 5, 6 are intended to provide the growth of a single crystal.